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C. E. BERRY Dec. 23, 1958 LINEARIZING ARRANGEMENT FOR STRING GALVANOMETER Filed 001;. 8, 1954 2 Sheets-Sheet 1 INVENTOR. CLIFFORD E. BERRY 4r, l d/7A4 4 ATTORNEYS RECORD/N6 AMPLIFIER FIG. 2.

v Dec. 23, 1958 c, E, BERRY 2,866,160

LINEARIZING ARRANGEMENT FOR STRING GALVANOMETER Filed Oct. 8, 1954 2 Sheets-Sheet 2 SPACl/VG =LENGTH TIMES K LENGTH FIG. 5.

INVENTOR. CLIFFORD E. BERRY A TTQRNEVS on the string element.

United States PatentO LINEARIZING ARRANGEMENT FOR STRING GALVANOMETER Application October 8, 1954, Serial No. 461,086

1 Claim. (Cl. 324-154) This invention relates to improvements in string galvanometers, and it has particular reference to an arrangement for causing string galvanometers to have a substantially linear current-deflection response.

When large amplitudes are required from a string galvanometer, such as when the galvanometer is employed in a direct writing oscillograph, the string element must be deflected large distances. This may be achieved by employing a flexible conductor as the string element and supporting it by springs at its two ends so that the central portion of the string can be deflected transversely a large distance in either direction in order to provide large amplitudes of motion. I 7

Application Serial Number 461,088, which was filed on October 8, 1954, by Lewis B. Browder discloses and claims such a string galvanometer.

As the string is deflected in either direction, the springs which support the string permit the end supports for the string to move closer together. However, the spring supporting system does not maintain the tension on the string exactly constant, and the tension on the string increases in accordance with the amount of deflection of the string. The deflection of the central portion ofthe string. is directly proportional to the current which flows through it and inversely proportional to the tension on it., Hence,

variations in the tension afiect the linearity of the galvanometer.

In accordance with the present invention, non-linearity in the current-deflection response of the galvanometeris corrected by causing the magnetic field to be non-uniform in such a way as to compensate for changes in the tension The non-uniform magnetic field is provided by employing pole pieces having recessed or cut out areas arranged so that the length of the conductive string upon which the magnetic field operates increases as the central portion of the string is deflected in either direction.

The spring supports fonthe string are 'leaf. springs which provide resilient support along the direction of the string and which restrain movement of the ends of the string in all other directions. Preferably, a. pair of pins are affixed to the ends of the respective leaf springs and the string is looped around the respective pins in a 1954, by Clifford E. Berry.

The invention is explained in more detail with reference to the drawings, in which:

Fig. 1 is a simplified disclosure of string galvanonieter recording apparatus;

Fig. 2 is an elevation view of one embodiment of the invention;

Fig. 3 is a sectional view along line 3-3 of Fig. 2;

Fig. 4 is a perspective view of one of the springsupports;

Fig. 5 shows how the string element of the galvanometer may be attached to the spring support;

Fig. 6 shows an alternative arrangement for attaching the string element of the galvanometer to the pin which is carried by the spring support;

Fig. 7 is a plan view of one-half of one of the pole pieces showing the cut out area for correcting non-line- 1 arity in the response of the galvanometer;

Fig. 8 is an elevation view of onehalf of the upper pole piece; and r Fig. 9 is an elevation view of one-half of the lower pole piece.

Fig. 1 is a simplified disclosure of a large amplitude string galvanometer. The string element of the galvanometer comprises a flexible conductor 10 which is supported at its ends by a pair of springs 12 and 14. The springs are arranged to provide resilient support for the string element along the direction of the string, and they serve to restrain movement in all other directions.

Amagnetic pole piece 16 and a pair of members 18 and 20 forming the other pole piece arelocated on 0p posite sides of the string element and they provide a magnetic field which is disposed perpendicularly with respect to the length of the string element. The'upper pole piece is in two parts so as to permit a recording medium to pass between them and adjacent the string element of the galvancmeter.

A conductive guide or anvil 22 islocated in the space between the two upper pole pieces. The anvil has an edge 22A which extends perpendicularly with respect to the string element, and a current-sensitive recording paper 24 moves over this edge of the anvil so that it extends between the conductive anvil and the string element of the galvanometer in the plane of motion of the string. The ends of the string element are connected to a signal source such as a recording amplifier 26. The electric current which flows through the string element causes its central portion to be deflected in accordance with the magnitude of the signals provided by the signal source. .Co-pendmg application Serial Number 445,518, which was filed on July 26, 1954, by Norton W. Bell discloses one suitable recording amplifier.

A source of electric current 28 has one terminal connected to the string element of the galvanometer, and it has the other terminal connected through a current limitmg resistor 30 to the conductive anvil. Thus, current flows through the current-sensitive recording paper at the intersection between the string element and the line formed by the edge of the conductive anvil. Hence, the electric current which flows between the string element and the conductive anvil causes a trace to be recorded on the current-sensitive recording paper in. accordance with the movements of the string element.

In the apparatus of Fig. l,the deflection of the string is directly proportional to the tension on it, so that variationsin the tension on the string element afiect the linearity of the galvanometer. The arrangement illustrated has a current-deflection response which varies 10 to 15% from linear due to the fact that the tension on the' string is a function of the deflection of the string.

Figs. 2 to 9 show the details of one version of recording apparatus which has good linearity, and it is capable of recording traces having one inch peak-to-peak amplitude up to a frequency of 250 cycles per second. I

As before, the string element is a flexible conductor 40 which is supported by a pair of leaf springs 42 and 44;

The ends of the leaf springs are notched, and cylindricalof the springs. respective pins in a loose fit, as illustrated in Fig. 5 or in Fig. 6, and the ends of the string element extend approximately perpendicularly with respect --to thedircction of movement of the strings to form a pairof pigtails A and 40B-Which-are connecte'd to a pair of'terminalsand 52 for coupling thewst-ri'ng to asignal The terminals 50 and 52 and the springs iz and-44am mounted on-arigid bar 53 which is aifixed source.

to the magnet yoke. The springs and the tcrrninalsare insulated from therest of the structure.

With such an arrangement the pigtail end portions 40A and 40B are subjected primarily to torsional stresses only, and hence they-havea long'life. The pigtailspro vide' a'reliable electrical connection to the string, yet they exert negligible forcesupon the string itself.

Preferably; the leaf springs are mounted to extend along a direction which is perpendicularv to both the direction of the magnetic field and the direction along which the string extends, as shown in Figs. 2 and 3. With such an arrangement, the leaf springs provide resilient'support along the direction of the string and they restrain'movement of the ends of the string in all other directions. Also, flexure of the springs does not displace the string from its central location between the two polepieces, and'hence' the pressure of the string on the recording paper does not change as, the string is deflected transversely.

The magnetic field for the string element is provided bya magnet54 having one pole piece which comprises a-pair of members 56 and 58, and having a yoke 60 interconnectingthe other pole of the magnet and another pelepiece' comprising a pair of members 62 and 64.

The-two' memberswhich constitute each pole piece are located in the same plane and they are disposed end to end with a small spacing being provided between the adjacent 'ends.

Aconductive anvil 66'is located between the members 62 and 6'4" of the upper pole piece, and it is provided with an edge 66A which extends perpendicularly with respect to the length of the string element. A suitable recording medium 68, such as current-sensitive paper, is passed over the edge of'the conductive anvil between the anvil and the string element of the galvanometer so that the recording medium is located approximately in the plane of movement of the string.

Fig. 3 illustrates. the deflection of the string from its central location between. the two pole pieces. The deflection of the center of the string element is directly proportionalvto the magnetic field strength andto the electric current which. flows through the stringelement of the galvanometer, and it isinversely proportional to the tension, in the tring element. The spring supports 42and 44 are resilient along,the.directionalong-which the stringlextends, but the :tension on.the:string increases asit is deflected from .itscentral location. This-increase in tension on the string, element. tends to cause nonlinearity in the current-deflection response of the galvanometer.

This is corrected in accordance with the present invention by providing recessed or cut out areas in the central portions of the pole pieces so that the length of the string upon which the magnetic field operates increases as the central portion of the string is deflected in either direction. One suitable arrangement is illustrated in Fig. 3 wherein the adjacent ends of the two members forming one pole piece have opposed cut out. areas" 70 and 72 of parabolic shape. With such an arrangement the other pole piece should'have a'cut out area which corresponds to that of the pole piece shownon Fig.3. Figs. 7, 8 and 9 show thedetails 'of'mernbersfor form'- ing suitable pole pieces. Fig. 7 shows the parabolic shape of the cut out area, Fig. 8 shows.-: one"half of-' the upper pole piece, and Fig. 9 shows one-half of the lower pole-piece.

The shape of the cut out areas 70 and 72 is defined to the first order by the formula where K times the length of the pole piece of the magnetequalsthespacing between the edges of the opposed parabolas for any selected values of Y, K is the value of K at the center of the pole piece where Y equals zero, C is thenon-linearity coeflicient which can be determined experimentally, and Y is the lateral distance in either direction from the center of the pole piece. Setting the spacing between the edges of the opposed parabolas to be 0.188 inch where Y'is 0.5 inch, setting the length of the pole piece to be 3.5 inches, and assuming that C=2.48 (which was determined experimentally for the apparatus of Figs. 2 and 3), then where Y is 0.5,

Using this value for K. in the :formula given above, it will be. found that K =.244. for these parameters. Then the formula which definesthe shape of the cut out areas forthese parameters is.

The non-linearity, coelficient C is the coefficient in the expressionwhere T is thetension on the string, T is the tension on the string when 'it is not deflected, and Yis the lateral displacement of the central portion of the string.

The coefiicient C can be determined experimentally from current-deflection measurements conducted with magnet pole pieces which are not cut out to correct for the nonlinearity by 'ascertainingits value in the following equation The above equations do not take into account the fringingleffects of the magnetic field in the cut out areas, and hence the cut out areas should be slightly larger than calculations indicate in-order to obtain the best linearity.

Typical dimensions are shown on the drawings'for a recording arrangement wherein the magnetic field is 16,000 Gauss. The pole pieces-are made of cold-rolled steel, the':leafsprings of beryllium copper (Berylco 25 Alloy), which isformed in the shape indicated in Fig. 4 and then age=hardened for two hours at 600 F., and the string element is composed of beryllium copper having a ductor and inversely: proportional. to the tension in the flexible conductor, the improvement whichcomprises'a pair of opposed magnetic polenp ieces .located on opposite sides of the flexible conductor, with the central portion of each polepiece being .cut out to provide recessed areas defined by opposedconcave surfaces in the opposing surfaces of the pole pieces, the recessed areas having substantially the shape of .two parabolas with a common axis of symmetry parallel to the undeflect'ed conductor element, the recessed areas being widest in a direction parallel to the'ccnductorelement at'th'e' center of the pole pieces and decreasing in width to either side of the center of the pole pieces, whereby the length of the flexible conductor element upon which the magnetic field operates increases as the central portion of the flexible conductor is deflected in either direction, to thereby compensate for variations in the tension in the flexible conductor and provide substantially linear current-deflection response, the width of the recessed areas as measured parallel to the undeflected conductor element varying according to the relation spacing between the edges of the opposed parabolas for any selected value of Y, K is the value of K at the center 6 of the pole pieces where Y=0, C is a constant which is determined by the overall structure of the apparatus, Y is the lateral distance in either direction from the center of the pole piece.

References Cited in the file of this patent UNITED STATES PATENTS Heiland May 11, 1954 

